如何使这个查询在LINQ to Entities中工作？

Question

我有下面的代码：

private static bool DoesColValueExist<T>(IQueryable dataToSearchIn, string colName, string colValue) 
{ 
    int noOfClients = 1; 
    Type type = typeof(T); 
    if (colValue != "" && colName != "") 
    { 
     var property = type.GetProperty(colName); 
     var parameter = Expression.Parameter(type, "p"); 
     var propertyAccess = Expression.MakeMemberAccess(parameter, property); 
     Expression left = Expression.Call(propertyAccess, typeof(object).GetMethod("ToString", System.Type.EmptyTypes)); 
     left = Expression.Call(left, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes)); 
     Expression right = Expression.Constant(colValue.ToLower(), typeof(string)); 
     MethodInfo method = typeof(string).GetMethod("Equals", new[] { typeof(string) }); 
     Expression searchExpression = Expression.Call(left, method, right); 


     MethodCallExpression whereCallExpression = Expression.Call(
      typeof(Queryable), 
      "Where", 
      new Type[] { type }, 
      dataToSearchIn.Expression, 
      Expression.Lambda<Func<T, bool>>(searchExpression, new ParameterExpression[] { parameter })); 
     var searchedData = dataToSearchIn.Provider.CreateQuery(whereCallExpression); 
     noOfClients = searchedData.Cast<T>().Count(); 

     if (noOfClients == 0) 
      return false; 
     else 
      return true; 
    } 
    return true; 
} 

它与LINQ到SQL但LINQ到实体，我得到的错误：

LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.

Answer 1

LINQ到实体不支持的ToString（）方法。我也不确定对于不是字符串的类型使用字符串比较是否是一个好主意。但并非所有人都失去了。我想出了以下解决方案：

public partial class MyEntity 
{ 
    public int ID { get; set; } 
    public int Type { get; set; } 
    public string X { get; set; } 
} 

public class MyContext : DbContext 
{ 
    public DbSet<MyEntity> Entities { get; set; } 
} 

class Program 
{ 
    static void Main(string[] args) 
    { 
     Database.SetInitializer(new DropCreateDatabaseIfModelChanges<MyContext>()); 

     using (var ctx = new MyContext()) 
     { 
      if (!ctx.Entities.Any()) 
      { 
       ctx.Entities.Add(new MyEntity() { ID = 1, Type = 2, X = "ABC" }); 
       ctx.SaveChanges(); 
      } 

      Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.X, "aBc")); 
      Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.X, "aBcD")); 
      Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.Type, 2)); 
      Console.WriteLine(DoesColValueExist(ctx.Entities, e => e.Type, 5)); 

     } 
    } 

    private static bool DoesColValueExist<TEntity, TProperty>(IQueryable<TEntity> dataToSearchIn, Expression<Func<TEntity, TProperty>> property, TProperty colValue) 
    { 

     var memberExpression = property.Body as MemberExpression; 
     if (memberExpression == null || !(memberExpression.Member is PropertyInfo)) 
     { 
      throw new ArgumentException("Property expected", "property"); 
     } 

     Expression left = property.Body; 
     Expression right = Expression.Constant(colValue, typeof(TProperty)); 
     if (typeof(TProperty) == typeof(string)) 
     { 
      MethodInfo toLower = typeof(string).GetMethod("ToLower", new Type[0]); 
      left = Expression.Call(left, toLower); 
      right = Expression.Call(right, toLower); 
     } 

     Expression searchExpression = Expression.Equal(left, right); 
     var lambda = Expression.Lambda<Func<TEntity, bool>>(Expression.Equal(left, right), new ParameterExpression[] { property.Parameters.Single() }); 

     return dataToSearchIn.Where(lambda).Any();     
    } 
} 

关于它的好处是，它更比型基于字符串的解决方案安全 - 参数的值必须是相同属性的值。该属性又必须是作为第一个参数传递的IQueryable'1的泛型类型的实体的成员。另一个有用的事情是，在针对此方法进行编码时，当您开始为第二个参数键入lambda表达式时，intellisense将向您显示实体的成员。在方法本身中，当我对属性值和请求值调用.ToLower（）以使比较大小写不敏感时，我为字符串类型添加了一个异常。对于非字符串类型，这些值按“原样”进行比较，即不进行任何修改。 上面的例子是完整的 - 您可以将其复制并粘贴到控制台应用程序项目（尽管您需要引用EntityFramework.dll）。 希望这有助于。

Answer 2

试试这个：

private static bool DoesColValueExist<T>(IQueryable dataToSearchIn, string colName, string colValue) 
{ 
    int noOfClients = 1; 
    Type type = typeof(T); 
    if (colValue != "" && colName != "") 
    { 
     var property = type.GetProperty(colName); 
     var parameter = Expression.Parameter(type, "p"); 
     var propertyAccess = Expression.MakeMemberAccess(parameter, property); 
     Expression left = property.PropertyType == typeof(string) ? propertyAccess : Expression.Call(propertyAccess, typeof(object).GetMethod("ToString", System.Type.EmptyTypes)); 
     left = Expression.Call(left, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes)); 
     Expression right = Expression.Constant(colValue.ToLower(), typeof(string)); 
     MethodInfo method = typeof(string).GetMethod("Equals", new[] { typeof(string) }); 
     Expression searchExpression = Expression.Call(left, method, right); 


     MethodCallExpression whereCallExpression = Expression.Call(
      typeof(Queryable), 
      "Where", 
      new Type[] { type }, 
      dataToSearchIn.Expression, 
      Expression.Lambda<Func<T, bool>>(searchExpression, new ParameterExpression[] { parameter })); 
     var searchedData = dataToSearchIn.Provider.CreateQuery(whereCallExpression); 
     noOfClients = searchedData.Cast<T>().Count(); 

     if (noOfClients == 0) 
      return false; 
     else 
      return true; 
    } 
    return true; 
} 

基本上，如果属性是字符串，则不会调用ToString()方法。

希望它有帮助。